Multiplex carrier current telephony



April 3, 1951 A. FROMAGEOT ETAL 2,546,994

MULTIPLEX CARRIER CURRENT TELEPHONY Filed Jan. 20, 1948 2 Sheets-Sheet 1 @Q91 PRENOM/LA R FISQ?? HUDULA TOR 'H s H 4 INVENTORS ANTOINE FROMAGUET MARC A LALANDE BY ROBERT A. REDARD Anm April 3, 1951 Y A. FROMAGEOT ETAL 2,546,994 v MULTIPLEX CARRIER CURRENT TELEPHONY Filed Jan. 20, 1948 2 Sheets-Sheet 2 L 1L x RNVENTORS ANTa/NE FRoMAaaET MARC A. LALANDE BY Rassm A. RED/mp ATTORNEY 2 Patented Apr. 3, 1951 UNITED STATES PATENT oFFlcE ternational Standard Electric Corporation, New York, N. Y., a, corporation of Delaware Application January 20, 1948, Serial No. 3,208" In France May 15,- 1945 Section 1, Public Law 690, August 8,- 1.946 Patent expires May 15, 1965 The present invention relates to transmission systems and particularly to carrier current multiplex telephone systems.

According to Certain features of the invention, use is made of a preliminary modulation of all the channels at one same selected frequency of a value higher than the maximum frequency transmitted over the line, this value being such that the products of this first modulation have a sufficient frequency spacing from the voice frequencies to permit use of a simple modulator for this first modulation.

' According to another feature of the invention, the use of a first modulation frequency considerably higher than the maximum frequency transmitted over the line makes it possible to employ simple modulators for a second modulation that serves to bring back the various channels into the transmission band stepwise, since the modulation currents, applied to these modulators falloutside of the band of frequencies transmitted over the line.

According to other features of the invention, at the output of the first modulators that all employ the same carrier, use is made of filters which serve for eliminatingone side band and which have a steep cut-off in the vicin'ty Vof the carrier frequenoy but which do not require a rapid cut-'off to the frequencies that correspond to the high voice frequencies, since the voice currents of the various channels are limited in advance by a lowpass filter of easy construction.

Accordingv to other features of the invention, the low frequency filters located ahead of the 'rst modulation are eliminated and use is made at the output of the rst modulators of filters having a pa'ss band Vstrictly limited to the Width of the voice bands and consequently of the side band to be transmitted.

The invention is described hereunder forcertain examples' of embod'ment and with reference to the appended drawings, in which:

Fig. 1 is a simplified schematic of an embodiment.

Fig. 2 is a schematic 0f' an embodiment in greater detail.

Fig. 3 is a schematic of another embodiment in greater detail.

' Referring to Fig. l, this shows an overall schematic of va carrier current system incorporatingV features of the invention. Reference number l' indicates a voice frequency subscribers line terv 7 Claims. (C1. 179-15) followed by' a filter 3'. d'indicates a second modu; lator, and 5 a filter terminating at the lin'e. To make the description clearer, it is assumed that the band o'f frequencies transmitted over ther line extends from 12' to 6'0 kilocycles and that' theban'd of voice frequencies to be transmitted extends from 300' to 3400 cycles'. Underthese conditions, the characteristics' of the various elements .of a system like `that shown .in Fig. 1 would be astfol'- lows: At the transmitting end, the low-pass' filter 2 limits the transmitte'dvoice frequencies to 3400 cycles. The r'st modulator', or premodulator, 3 is fed by a carrier frequency', e. g. of between 80 and 'kilccycles tnatis hignertnan the masimumfrequency to be transmitted, e. g'. of 60' kilocycles.

lVIGlliBLi/Ol''may be" Of 9J Simple type, e. g'. a S- called branched bridge modulator as shown in Fig. 2.

As an'iatter of fact, the useful modul'ati'on'nroducts are very Widely separated in frequency from the modulating voice frequencies, and the latter are accordinglyV eliminated in the subsequentr circuits vvith'out special precautions'.

Y Modulator 3 is followed by a quartz lter'vvl'iicl eliminates one of the modulation side bands' and whiih completes the action of 10W-pass filter 2', limiting the band of voice frequencies to 3400 cycles in the'selecte'd example. The cut-off characteristic has tov be sufficiently' abrupt on the side of the pass band nearest the' carrier current in order to effect `suitable elimination of the second. band, but the cut-off is" not necessarily rapid at the other end of the pass band on which the ac'- tion of'low frequency low-pass lte'r 2 is' considerable.

Thisl filter is followed by a second modulator il' which serves for bringing" back the selected side band into the range of frequencies transmitted' over thel line', the'carrier frequencies 'of the second' modulators for the various lines being staggered bye kilocycles' in the'selected example'. The selection of a first Vnicflu-lation frequency that is high' as comparedv witlithe highest frequency' trans'- mitte'd over vthe linek kpermits the 'use of simple modulators; Indeed itis' no longer necessary to employ balanced modulators for eliminating in their output circuit's'the' frequencies of the applied signals, since' these have frequencies sufficiently far removed from the useful modulation products. The output filter 5, and also the modulator, may consequently be of a simplified type, since there is no need to separate closely adjoining frequencies; y

Use can accordingly also be made Vin this case 3 of simple modulators, such as bridge modulators, that do Inot stop the modulation frequencies.

The transmission systems according-to the invention permit the construction of simple equipment. Y

The carrier frequency P of 80 to 150 kcs. hasV been determined so that the blocking of the input signals of a modulator or of a demodulator may take place without difiiculty inthe filter disposed back of that element, and without necessitating the use of the ring modulators that are generally employed in carrier current systems and that require link transformers. For this purpose, Po has to be such that the frequencies of the modulation products are definitely different from those of the modulating signals, e. g. of theorder of 80 kcs. or more. This frequency Po hasalso such a value as to facilitate the generation for the second modulators of the channel carrier frequencies of values Poiin, n being any-integer from say 3 to 14 and the carrier frequency generating equipment has to be simple and sturdy. A known method that gives complete satisfaction consists in seeking these carrier frequencies among the harmonics of a fundamental oscillation. In using this method, it is advisable to em- Y ploy channel carrier frequencies P01411, that are less than about 140 kcs., and this sets for P0 a top limit of about 84 kcs. in the case of channel carrier frequencies of the form P+4n.

The Value ofA 80 kcs. taken in the described example of embodiment corresponds to a premodulationfilter I2 that is easy to construct by means of quartz crystals, which make it possible to obtain the required non-distortion in the transmitted band without the intervention of correctors.' This quartz filter comprises a. lattice section containing a single crystal in each arm. It is of the unilateral` type, only comprising infinite attenuation frequencies on one side vof the band of transmitted frequencies.

The voice frequency and modulation band pass or low-pass filters are ladder filters of the conventional type.

It is to be noted that the modulators employed are' all ofthe branched bridge type. This type of modulator has the advantage of great simplicity, but it has two drawbackswhich are here without eiect on account of the dimensioning of the elements of the system, These modulators d'o not stop the modulation frequencies, but this is of no importance owing to thesuitable selection of carrier P0; also if used without special precautions, they have an output about 6 db. less than the usually employed ring modulator, but their output can be considerably increased to a yjalue comparable to that of a ring modulator by suitable arrangement of the elements disposed on either side of the modulators. For this, it is suflicient for the circuits that receive the modulated signals to have a high impedance for the modulation signals, and for the circuits that furnish the modulation signals, to have a high impedance for the modulated signals.A In case a modulator is placed directly at the input or output of a filter, the circuits of the filter are arranged so as to comply with this condition.

The conditions required from filters in a multichannel telephone system relate to the non-distortion of amplitude of the transmitted signals, and to the non-transmission of useless and disturbing signals, namely, signals that correspond to' voice frequencies outside of the frequency band assigned to ar channel, usually 4 kcs.; signals proceeding from unused modulation bands; and

. filter I2.

, in each arm; on account vof the high value of thecarrier frequency P0, the crystals employed are of limited size and make it possible to house thisA image frequency signals foi the demodulaton operations.

The two last conditions can only be satisfied by means of filters disposed back of the modulating element and in front of the demodulating element, but the two first mentioned conditions can be met by means of correctors and filters inserted in the Voice frequency circuits, for example, or in any other suitable location.

Fig. 2 shows a first example of a system designed according to the features of the invention.

Referring to this Fig. 2, it comprises at the transmitting end a filter II'I that helps to limit the transmitted Voice frequencies to 3400 C. P. S.. the transmission curve required from this filter being dependent on that of the two subsequent lters Whose action it only serves to complete; a branched bridge premodulator I I fed by a carrier frequency P0 denitely higher than 60 kcs., e. g. of the order of to 100kcs.; a filter I2 which stops the unutilized premodulation band (top band) and also helps to limit the transmitted voice frequencies to the band of 300 to 3400 C. P. S., this filter being arranged in such a Way that it does not introduce distortions in the amplitude of the transmitted signals, so that it does not require the presence of additional corrector elements; a branched bridge channel modulator I3 fed by one of the carrier frequencies Pi=P0I-4n, n being a whole number comprised .between 3 and 14 inclusive; and a channel filter I4 Which suppresses the unused modulation band and also completes the action of premodulation This filter is also arranged in such a way that it does not introduce distortion in the amplitude of the transmitted signals.

At the receiving end, the illustrated system comprises a channel filter I5 that eliminates the image frequencies from the channel demodula'` tion and helps to complete the action of the filterl that precedes the second demodulaton; a lbranched bridge channel demodulator I6 fed by one 0f the carrier frequencies Pi=P0-|-4n, n being a Whole number comprised between 3 and 14 inelusive; a second demodulaton filter I'I Whichv eliminates the image frequencies from the sec-f ond modulation and helps to limit the .received voice frequencies to 340,0 C. P. S.; a second branched bridge demodulator I8 fed bythe car.` rier frequency P0; anda low frequency filter I9, associated to a low frequency amplifier, that helps to limit the received voice frequencies to 3400 C.P. S.

For the purpose of simplifying the drawing, the filters at the receiving end are shown in the schematic as being identical with the filters at the transmitting end.

A second embodiment shown in Fig. 3 com-lY lows the first modulator, like the one that pre-- cedes the second demodulator, consists of a lattice section that comprises two quartz crystals filter in the same volume as the crystal filter of the previously described system. The advantages of this embodiment, which has transmission characteristics comparable to those of the previously described system, are to be found in its small number of members and small size, thus making it possible to considerably reduce all the expense connected with the insLallation of the system.

It is evident that the present invention is by no means limited to the described examples of embodiment, and that the same are capable of variants and modifications without departing from the scope of the invention.

What is claimed is:

1. In a multichannel carrier current communication system, a common carrier current transmission line for the transmission of signals having frequencies lying Within a predetermined band of frequencies, a plurality of channels coupled to said common transmission line, each channel transmitting signals having frequencies lying within a given frequency range, a premodulation low pass filter in each channel to pass signals having frequencies lying within said given frequency range, a first modulator in each channel coupled to the output of its respective premoclulation filter to provide first upper and low sidebands, means for supplying to all said iirst modulators a common carrier wave having a frequency higher than any of the frequencies lying within said predetermined band of frequencies,

first band-pass filter means in each channel to select one of said first sidebands and having a sharper cut-off characteristic at that end of `the pass-band nearest the carrier wave than at the other end, a second modulator in each channel coupled to the output of its respective first band-y pass filter to provide second upper and lower sidebands, means for supplying to said second modulators carrier Waves having frequencies different for each channel, second band-pass filter means to select one of said Second sidebands in each channel, and means for coupling the output of said second band-pass filter means to said common transmission line.

2. In a multichannel carrier current communication system, a common carrier current transmission line for the transmission of signals having frequencies lying within a predetermined band of frequencies, a plurality of channels coupled to said common transmission line, each channel transmitting signals having frequencies lying Within a given frequency range, a premoduiation low pass filter in each channel to pass signals having frequencies lying within said given frequency range, a rst modulator in each channel coupled to the output of its respective premodulation lter to provide first upper and` low sidebands, means for supplying to all said first modulators a common carrier wave having a frequency higher than any of the frequencies lying within said predetermined band of frequencies, first band-pass filter means in each channel to select said first lower side band and having a sharper cut-ofi characteristic at the higher end of the pass band than at the lower end, a second modulator in each channel coupled to the output of the respective first band-pass filter to provide second upper and lower sidebands, means for supplying to said second modulators carrier waves having frequencies different for each channel, second band-pass filter means to select the lower second sideband in each channel, and means for coupling the output of said second band-pass lter means to said common transmission line.

3. In an electric carrier current communication system, a transmission line for the transmission of signals having frequencies lying within a given band of frequencies, a source of audio frequency signals, a low-pass filter coupled to said source, a first modulator coupled to said low-pass filter to provide first upper and lower sidebands, means for supplying to said rst modulator a first carrier wave having a frequency higher than any of the frequencies lying within said given band of frequencies, a first band-pass lter coupled to said first modulator to select one of said first sidebands and having a sharper cut-off characteristic at that end of the pass-band nearest said carrier wave, a second modulator coupled to said first band-pass filter to provide second upper and lower sidebands, means for supplying to said second modulator a second carrier wave, a second band-pass filter coupled to said second modulator to select one of said second sidebands, and means yfor coupling the output of said second band-pass filter to said common transmission line.

4. In an electric carrier current communication system, a transmission line for the transmission of signals having frequencies lying within a given band of frequencies, a source of audio frequency signals, a low-pass filter coupled to said source, a first modulator coupled to said first lowpass filter to provide first upper and lower sidebands, means for supplying to said first modulator a first carrier wave having a frequency higher than any of the frequencies lying within said given band of frequencies, a, rst band-pass filter coupled to said first modulator to select said first lower sideband and having a sharper cut-off characteristic at the higher end of the pass band than at the lower end, a second modulator coupled to said first band-pass filter to provide second upper and lower side bands, means for supplying to said second modulator a second carrier wave, a second band-pass filter coupled to said second modulator to select the lower second sideband, and means for coupling the output of said second band-pass filter to said common transmission line.

5. A system according to claim 1 in which two stages of demodulation are employed at the receiver for each channel, the carrier frenquency used for the second stage of demodulation being the same as that used for the first modulation stage.

6. A system according to claim 3, in which these modulators (or demodulators) used in both stages of modulation (or demodulation) are of the branched bridge type.

7. A system according to claim 3 in which the two stages of modulation (or demodulation) are separated by a crystal band-pass filter.

ANTOINE FRQMAGEOT. MARC ANDRE' LALANDE. ROBERT REDARD REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,270,385 Skillman Jan. 20, 1942 FOREIGN PATENTS Number Country Date 520,756 Great Britain May 2, 1940 204,088 Switzerland July 1, 1939 

